Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for dynamically adjusting an illumination of the subsurface during a seismic survey.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural sensors, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are mainly impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources is air guns which produce a high amount of acoustic energy over a short time. Such a source is towed either at the water surface or at a certain depth by a vessel. Acoustic waves from the air gun propagate in all directions. The emitted acoustic waves' typical frequency range is between 6 and 300 Hz. However, the frequency content of the impulsive sources is not fully controllable, and different sources are selected depending on a particular survey's needs. In addition, use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used is vibratory. Vibratory sources, including hydraulically-powered, electrically-powered or pneumatically-powered sources and those employing piezoelectric or magnetostrictive material, have been used in marine operations. A positive aspect of vibratory sources is that they can generate signals that include various frequency bands, commonly referred to as “frequency sweeps.” In other words, the frequency band of such sources may be better controlled, as compared to impulsive sources.
One example of such a vibratory source element is described in U.S. patent application Ser. No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, and entitled, “Source for Marine Seismic Acquisition and Method,” assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference.
Source arrays (i.e., a plurality of vibratory source elements) are now used in marine seismic acquisition because they more efficiently generate acoustic energy. The source array can be towed at a single depth or at variable depths, as would be the case for a curved source array. Dual or multi-level arrays are also sometimes used to reduce the effect of spectral notches due to destructive interference with surface reflections. A source array including source elements that are uniformly distributed at a single depth and operate synchronously with identical output spectra tends to create a symmetrical directivity radiation pattern.
With regard to FIG. 1, if a single-depth source array 110 including identical source elements 108 is towed behind a vessel 101, the energy emitted by the source array tends to be symmetrical, with equal amounts of energy radiated toward vessel 101 and toward the rear of this arrangement. Because the receiver array or streamer 105, which typically includes hydrophones 106 (it can also include particle motion sensors or any sensor configured to detect seismic signals), is also towed behind vessel 101, much of the acoustic energy source array 110 radiates is not helpful for illuminating a subterranean target that might be a hydrocarbon reservoir.
This may become more problematic when trying to image a dipping reflection event 118, for example, the flank of a salt dome below the ocean bottom 116. As can be seen in FIG. 1, the energy emitted that follows ray path 112 does strike the target, i.e., the dipping reflection event 118. Its reflection path reenters the water at an unfavorable angle so streamer 105 does not receive the acoustic energy. Thus, this energy is wasted. Note that only energy following ray path 114 arrives at streamer 105.
Thus, there is a need to provide a method that directs and maximizes acoustic energy in a preferred direction (beam steering) to better illuminate a target of interest so that most of the reflected energy is directed toward the streamer spread.